Gas turbine engines typically include a combustor in which carbonaceous fuel is combusted with an oxidant such as air to produce hot gases of combustion. Frequently, the hot gases of combustion are diluted with cooler air which together are directed through a turbine nozzle and then against a turbine wheel or rotor. This is done because it is known that high operating temperatures in those parts of turbine engines subjected to the hot gases of combustion are potentially damaging, and large temperature gradients that might otherwise result cause large internal stresses due to differences in thermal expansion. Additionally, the high operating temperatures might otherwise require the use of more expensive materials in constructing gas turbine engine components in order to withstand fatigue. Because of such factors, it has been customary to inject dilution air into the hot gases of combustion at a point upstream of the turbine wheel or rotor and the turbine nozzle.
Typically, it is desired to achieve a substantially uniform circumferential mixing of the dilution air with the hot gases of combustion in order to produce a desirable temperature profile. In an optimal case, there will be a complete mixing of the dilution air with the hot gases of combustion such that a uniform temperature of a stream of combined gases of combustion and dilution air is achieved which means that the operating temperature can be adequately regulated by controlling, through suitable design parameters, the amount of dilution air in proportion to the gases of combustion. At the same time, severe temperature gradients will be nonexistent because all parts of the gas stream being applied to the turbine nozzle and thus to the turbine wheel or rotor are at substantially equal temperatures.
Perfect circumferential mixing cannot be obtained in practice although it may be more closely approached in large sized turbines. This follows because the size of the components is such that there is substantial residence time of the combustion gases and dilution air in a large combustor prior to their arrival at the turbine nozzle so as to allow fairly thorough mixing. However, for small sized turbines, the residence time is ordinarily extremely short such that adequate mixing will not necessarily occur.
In order to attain desired temperature gradients within a small combustor, cooling air can be injected into the combustion chamber through a plurality of dilution air holes. As will be appreciated, the dilution air holes will make it possible to be able to maintain an acceptably uniform temperature gradient within the combustion chamber.
However, there is another problem intertwined with the use of dilution air holes. During the combustion process, there is a tendency for carbon build-up to occur in the combustor as a result of a number of factors. For instance, fuel maldistributions may result from contamination such as gum build-up in the fuel injectors after considerable periods of service and, when this happens, carbon build-up may result and operating efficiency will necessarily be adversely affected. But even more importantly carbon build-up is undesirable since pieces of carbon may break off and be swept throughout the engine. Such carbon can cause erosion of engine parts and reduce the life of the engine.
As for air blast fuel injectors, the air passageways are susceptible to plugging by one or more carbon lumps formed when carbon breaks away from a carbon build-up within the combustion chamber. When carbon breaks away, a carbon lump may subsequently fall out of the combustion chamber through a dilution air hole into the combustor air flow annulus between the combustor housing and the wall defining the combustion chamber, typically, as the gas turbine engine is being shut down. In a subsequent restart, the carbon lump can be carried forward by compressed air flowing through the combustor air flow annulus until it eventually lodges in the air passageway of one of the fuel injectors.
Obviously, if the air passageway of the air blast fuel injector is blocked, poor fuel atomization will occur which can be very destructive inasmuch as hot streaks can be formed which can seriously damage the turbine nozzle. When this occurs, there will be an even more accelerated carbon build-up which can result in still additional carbon lumps with consequent rapid erosion of the turbine nozzle and turbine wheel or rotor.
The present invention is directed to overcoming one or more of the foregoing problems and achieving one or more of the resulting objects.